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Chapter 2
IR Detectors For
Thermographic Imaging
IR Cameras
Thermographic imaging is accomplished
with a camera that converts infrared
radiation (IR) into a visual image that
depicts temperature variations across
an object or scene. The main IR camera
components are a lens, a detector in
the form of a focal plane array (FPA),
possibly a cooler for the detector,
and the electronics and software for
processing and displaying images (Figure
1). Most detectors have a response curve
that is narrower than the full IR range
(900–14,000 nanometers or 0.9–14µm).
Therefore, a detector (or camera) must
be selected that has the appropriate
response for the IR range of a user’s
application. In addition to wavelength
response, other important detector
characteristics include sensitivity, the
ease of creating it as a focal plane array
with micrometer size pixels, and the
degree of cooling required, if any.
In most applications, the IR camera must
view a radiating object through the
atmosphere. Therefore, an overriding
concern is matching the detector’s
response curve to what is called an
IR In
NIR
MWIR
LWIR
Detector Cooling
Digitization
Optics
atmospheric window. This is the range of
IR wavelengths that readily pass through
the atmosphere with little attenuation.
Essentially, there are two of these
windows, one in the 2–5.6µm range, the
short/medium wavelength (SW/MW) IR
band, and one in the 8–14µm range, the
long-wavelength (LW) IR band. There are
many detector materials and cameras
with response curves that meet these
criteria.
Quantum vs. Non-Quantum Detectors
The majority of IR cameras have a
microbolometer type detector, mainly
because of cost considerations.
Microbolometer FPAs can be created
from metal or semiconductor materials,
and operate according to non-quantum
principles. This means that they respond
to radiant energy in a way that causes
a change of state in the bulk material
(i.e., the bolometer effect). Generally,
microbolometers do not require cooling,
which allows compact camera designs
that are relatively low in cost. Other
characteristics of microbolometers are:
• Relatively low sensitivity (detectivity)
• Broad (flat) response curve
• Slow response time (time constant
~12ms)
Video
Processing
Electronics
User Interface
User Control
Video Output
Digital Output
Synchronization In/Out
System Status
Figure 1. Simplified block diagram of an IR camera
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